What business that develops software can't afford $9 a month for an upgraded plan? The utility you get from this type of software far outweighs the cost.
Who wants their wireframes to be public projects? Maybe if you're working on UI's for open source projects? I can't think of any other use cases.
Precursor's unique value propositions seem to be aimed at problems I would guess few people have...
"The Free Plan includes 2 projects (limited to 300 objects) and 5MB of storage to get you started."